Water in comets

Other natural hydrates have been suggested during the past few decades. Miller (1974) concluded that hydrates of carbon dioxide were on Mars, and Pang et al. (1983) indicated that the E rings of Saturn were hydrates. Delsemme and Miller (1970), Mendis (1974), and Makogon (1987) suggested that hydrates exist in comets in particular, carbon dioxide and water in comets were combined in the form of hydrates. 

Aqueous chemistry is one of the oldest forces of change in the solar system. It started less than 20 million years after the gases of the solar nebula began to coalesce into solid objects.2 Water is also the most abundant volatile molecule in comets. On the earth, the oceans alone contain about 1.4 x 1021 kilograms or 320,000,000 cubic miles of water. Another 0.8 x 1021 kilogram is held within the rocks of the earth s crust, existing in the form of water of hydration. The human... 

Water can be found, in all three aggregate states, almost everywhere in the universe as ice in the liquid phase on the satellites of the outer solar system, including Saturn s rings and in the gaseous state in the atmospheres of Venus, Mars and Jupiter and in comets (as can be shown, for example, from the IR spectra of Halley s comet). The OH radical has been known for many years as the photodissociation product of water. 

The second important source for the hydrosphere and the oceans are asteroids and comets. Estimating the amount of water which was brought to Earth from outer space is not easy. Until 20 years ago, it was believed that the only source of water for the hydrosphere was gas emission from volcanoes. The amount of water involved was, however, unknown (Rubey, 1964). First estimates of the enormous magnitude of the bombardment to which the Earth and the other planets were subjected caused researchers to look more closely at the comets and asteroids. New hypotheses on the possible sources of water in the hydrosphere now exist the astronomer A. H. Delsemme from the University of Toledo, Ohio, considers it likely that the primeval Earth was formed from material in a dust cloud containing anhydrous silicate. If this is correct, all the water in today s oceans must be of exogenic origin (Delsemme, 1992). 

J In outer space, frozen water, or ice, has been found on the moon, on planets— particularly Mercury, Mars, Neptune, and Pluto—and in comets and clouds between stars in our galaxy. Recent explorations of Mars indicate that there may be liquid water underground on Mars.This means there could be microorganisms living there ... 

Depending on conditions, frozen substances in comet nuclei can be crystalline ices, amorphous ices, and clathrate hydrates (compounds in which cages in the water-ice lattice can host guest molecules). Compositions of the ices and associated organic materials in comets have been determined from both telescopic and spacecraft observations. Spectral line measurements of gases in a comet s coma allow the identification of molecules and radicals. An inherent difficulty in spectral measurements is that volatiles in the coma are commonly broken... 

Gravitational stirring of icy planetesimals by the giant planets could have sent many comets careening into the inner solar system, providing a mechanism for late addition of water to the terrestrial planets. Comets impacting the Earth and the other terrestrial planets would have delivered water as ice (Owen and Bar-Nun, 1995 Delsemme, 1999), whereas the accretion of already altered carbonaceous chondrite asteroids would have delivered water in the form of hydroxl-bearing minerals (Morbidelli el al., 2000 Dauphas et al., 2000). 

Noble gases may provide a constraint on the source of water and other volatiles. The abundance pattern of noble gases in planetary atmospheres resembles that of chondrites, perhaps arguing against comets. However, there are some differences, especially in the abundance of xenon. Relative to solar system abundances, krypton is more depleted than xenon in chondrites, but in the planets, krypton and xenon are present in essentially solar relative abundances (Fig. 10.11). This observation has been used to support comets as the preferred source of volatiles (even though measurements of xenon and krypton in comets are lacking). A counter-argument is that the Ar/H20 ratio in comets (if the few available measurements are accurate and representative) limits the cometary addition of volatiles to the Earth to only about 1%. 

Using different deposition rates, even a highly compacted form of amorphous solid water of density > 1 g/cm3 could be obtained at deposition temperature T < 30 K, which transforms gradually in the temperature range 38-68 K to the lower density form of density 0.94 g/cm3 [139, 140]. This transition was proposed to be at the origin of crack-formation processes in comets [141]. We note, however, that the formation of this high-density amorph at very low temperatures has been doubted [142, 143]. Only photolysis at 20 K induces a transition to a high-density amorph [143]. 

Carbonates are common in hydrous meteorites and hydrous IDPs, where they are believed to have formed by parent-body aqueous processing. Since simple models of cometary evolution involve no aqueous processing, carbonates were generally presumed not to occur in comets. However, carbonates have also been detected by infrared spectroscopy in the dust shell around evolved stars and in protostars, where liquid water is not expected (Ceccarelli et al. 2002 Kemper et al. 2002). Indeed, Toppani et al. (2005) have performed experiments that indicate that carbonates can be formed by non-equilibrium condensation in circumstellar environments where water is present as vapor, not as liquid. Detections of carbonates in other exosolar systems are reported by Ceccarelli et al. (2002) and Chiavassa et al. (2005). 

Water and other volatiles could have been supplied to Earth by comets and asteroids as part of the late veneer. The arguments for and against this hypothesis have recently been reviewed by Drake (2005). The D/H ratio measured in three comets to date is 2 x higher than on Earth, suggesting that comets could not have supplied more than 50% of Earth s water (Robert 2001). However, these comets may not be representative of objects colliding with the early Earth. If the Ar/H20 ratio measured in comet Hale-Bopp is typical, comets would have delivered 2 x 104 times more Ar than is presently found in Earth s atmosphere if they were the main source of Earth s water (Swindle Kring 2001). Consideration of the abundances of noble metals and noble gases led Dauphas Marty (2002) to estimate that comets contributed <1% of the Earth s water. It is unlikely that carbonaceous chondrites supplied most of the late veneer since these objects have different Os isotope ratios than Earth s mantle,... 

The question of the CO and CO abundances has not yet been solved. Delsemme and Combi explained the forbidden red oxygen lines as originating to a major part from dissociation of CO2 into CO and O. This would yield a COj production of 10 mol/s in Comet Kohoutek. The process would also explain the bulk of the CO observed in Comet West (Table 2). The authors expressed at that time the opinion that the presence of COj in cometary nuclei, though in somewhat lower abundance than water, is more probable than that of CO since the observed amounts of CO could also come from dissociation of CO. As model calculations have shown, it seems, however, difficult to understand that the observed abundances of CO ions originate from a pure CO2 nucleus (see p. 20). Moreover, Feldman and Brune ... 

Water ice seems to be the major constituent. Carbon-containing molecules (COj, CO) are of comparable, though lower abundances (up to 30%), It is not yet clear whether CO or CO or both are parent molecules of the nucleus. Average production rates for water in bright comets are 1(P -10 mol/s in 1 a. u. solar distance. 

Comets are rich in volatile elements, but they probably delivered no more than 10% of Earth s volatile inventory. There are several reasons for this. Comets have a very low impact probability with Earth over their dynamical lifetime ( 10 Levison et al., 2000), limiting the amount of cometary material that Earth could have accreted. In addition, if most of Earth s water was acquired from comets, it seems likely that Earth s noble gas abundances would be higher than observed by several orders of magnitude (Zahnle, 1998). Einally, water measured spectroscopically in comets differs isotopically from that of seawater on Earth, with the cometary D/H ratio being greater by a factor of 2 (Lunine et al., 2000). 

Figure 18 D/H ratios of several comets compared to the oceans (SMOW), planets, the solar nehula (PSN), and the interstellar medium. Low-temperature fractionation processes increase D/H. Jupiter and Saturn have compositions close to the original nehular composition, hut low-temperature formation of ice caused the enhancements seen in Uranus and Neptune (the ice giants) and comets. The discrepancy between the plotted LP comets and SMOW argues against these comets providing Earth with a major fraction of its water. Other comets, formed in warmer environments, near Jupiter, could he more similar to SMOW (source Huehner, 2002).

Mumma M. J., Weaver H. A., and Larson H. P. (1987) The ortho-para ratio of water vapor in Comet p/HaUey. Astron. Astrophys. 187, 419-424. 

The source of water for the formation of the hydrosphere is problematical. Some meteorites contain up to 20% water in bonded hydroxyl (OH) groups, while bombardment of the proto-Earth by comets rich in water vapour is another possible source. Whatever the origin, once the Earth s surface cooled to 100°C, water vapour, degassing from the mantle, was able to condense. Mineralogical evidence suggests water was present on the Earth s surface by 4.4 billion years... 

At the present time the balance of evidence is against a cometary origin for prebiotic carbon on Earth, for the same comets would also have delivered water to the Earth and yet the D/H ratio of the terrestrial oceans is different from that in comets. The more likely extraterrestrial input is from asteroids and meteorites, for there is evidence from both lunar and terrestrial samples that the late heavy bombardment event at 3.9 Ga (Section 6.4.1) contributed meteoritic material to the Earth at this time. 

Astrochemists recognize that the chemical species most easily observed in comets—simple molecules, ions, and free radicals such as H, O, OH, C+, CH+, CO+, C02+, C2, C3, CH, and CN-are probably formed by the photolysis of more complex molecules buried within cometary nuclei. The action of solar energy on a water molecule in a comet s nucleus is an example. When radiation energy (hv) from the Sun strikes a water molecule, it causes that molecule to dissociate into two or more parts. Among the possible reactions are the following ... 

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